Global navigation satellite systems (GNSS), such as the Global Positioning System (GPS) provide accurate position information when a sufficient number of satellites are in view. Hereafter, for convenience the invention will be described in the context of GPS, but it will be appreciated that other GNSS systems, such as the Russian GONAD system, or the European Galileo system can be employed.
The use of GPS systems in relation to vehicles running on a guideway gives rise to two problems. First, the accuracy of the location/positioning of the non-military available GPS signals is not sufficiently precise for ensuring accurate station stops, the location of switches on the guideway, locating the end of guideway etc. Second, GPS signals are not available in tunnels. This results from the nature of GPS systems, which are based on direct viewing of satellites in the sky. These problems prevent the use of readily available GPS-based positioning solution to exclusively determine train position in Railway/Transit applications.
Current solutions make use of RFID (Radio Frequency Identification) transponders to provide absolute location information to trains in the system including in a tunnel. This solution requires the installation of RFID transponders along the guideway and a reader on the train to provide position information. The position of the train between RFID transponders is determined by dead reckoning.
Other systems use inductive loop communication together with inductive loop cable transpositions (crossovers) to provide relative positioning information. On-board crossover detection equipment evaluates and counts the number of transpositions (crossovers) from the starting point of the inductive loop. This provides relative positioning/location information to the on-board system. The position of the train between transpositions(crossovers) is provided by dead reckoning.
These solutions are expensive both in terms of hardware and software and cannot be justified in certain applications. The limitation of the current GPS solution limits the option of providing a low cost “off-the-shelf” solutions based on readily available GPS devices.